1. Field of the Invention
The present invention relates to a vertically movable, marine working platform structure having vertically movable, groundable support frames and, more particularly, to a vertically movable, marine working platform structure having vertically movable, groundable support frames, which withstands external horizontal forces, such as the forces of wind and waves while the working platform structure is in tow or in grounding and setting-up operations or at work in spite of the weight-reduced support frames.
2. Description of the Prior Art
With a vertically movable, offshore working platform structure having vertically movable, groundable support frames, it is operated that while it is towed, the working platform itself is floated on the surface of the sea with the support frames maintained in a raised position, and when it is at work, the support frames are lowered into the sea water in a manner such that the structure is grounded on the sea bed and keeps standing by itself.
The material for and the sizes of the above working platform structure including the support frames are determined such that the working platform structure withstands preselected natural conditions while it is in tow or in grounding and setting-up operations or at work. Especially, when such a working platform structure is used for submarine oil field probes, it is necessary that the structure be made of a specially selected material to specially selected dimensions. Accordingly, the greater part of a working platform structure of this kind generally consists of high tension steel. Above all, the support frames consist of high tension steel of the highest grade. Under the circumstances, a great deal of effort is now going into the reduction of the weight of the support frames.
Each of the support frames consists of a plurality of cords arranged vertically in parallel with one another, a plurality of horizontal members both ends of each of which are joined to panel point portions of the cords, and a plurality of diagonal members both ends of each of which are joined to the panel point portions of the cords, therefore an utmost safety is obtainable when a guide provided to the working platform is situated at a point substantially corresponding to the location of the panel point portion of the cord. It then is that horizontal forces to be transmitted from the guide to the support frame can concentrate mainly onto the panel point portion and can be born substantially evenly by respective cords of the support frame through horizontal members and diagonal members, whereby concentration of loads can be effectively avoided upon the portion of the cord lying between two longitudinally adjacent panel point portions of the cord. On the other hand, when the guide is situated at an intermediate point on the cord lying between two longitudinally adjacent panel point portions, loads can tend to concentrate at such point on the cord, whereby a most dangerous condition can arise. Therefore, when the working platform structure is overtaken by a storm while it is being grounded and set up, the working platform is vertically moved so as to be retired to a safe place where the guide is in alignment with panel point portions of the same level of a support frame. However, the positions of foot cans attached to the lower ends of support frames are not in agreement with one another in practice with respect to the vertical axis of the working platform structure due to (a) the inclination of the surface of the sea bottom, (b) lack of uniformity of geologic formation in such portions of the sea bottom which are just under and around the foot cans and (c) unbalanced loads applied to the foot cans. On the other hand, it is necessary that a working platform be kept substantially horizontal. Consequently, with use of such a guide as being shorter than the distance between two vertically adjacent panel point portions of the cord, it practically is difficult to adjust the position of the working platform such that the guide is, in position, in alignment with the panel point portion.